Recording method and apparatus

ABSTRACT

An optical storage medium is disclosed. In general, the storage medium includes a substrate, with a first, expansion layer, a second, reflective layer bonded to the first layer, and a third, retention layer bonded to the second layer, opposite the first layer. An opto-electronic system for recording data is also disclosed, along with a method of manufacture of the storage medium.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 294,723 filed Jan. 10, 1989.

This application is a continuation-in-part of application Ser. No.153,288 which is incorporated herein by reference.

This application is related to applications having Ser. Nos. 152,519,152,690, 152,778 and 152,696.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to the field of recording media. Inparticular, one embodiment of this invention provides an erasableoptical storage media and write/read/erase mechanism therefor in whichdata may be recorded and erased in response to thermal effects and, inparticular in response to light.

2. Description of Related Art

Optical data storage media in the form of compact disks are well knownas an alternative to longplaying records and magnetic tape cassettes.The disks with which consumers are familiar are optical read-only disksand the common disk player is designed specifically for this type ofdisk. These disks have a reflective surface containing pits whichrepresent data in binary form. A description of these pits and how theyfunction is provided by Watkinson, "The Art of Digital Audio," FocalPress, Chapter 13.

Compact disks are currently produced by a pressing process similar tothe process used to produce conventional long playing records. Theprocess, referred to herein as the "mastering" process, starts by firstpolishing a plain glass optical disk. This disk has an outside diameterfrom 200 to 240 mm, a thickness of 6 mm and undergoes various cleaningand washing steps. The disk is then coated with a thin chrome film orcoupling agent, a step taken to produce adhesion between the glass diskand a layer of photo-resist, which is a photosensitive material. Data ona compact disk master tape are then transferred to the glass disk by alaser beam cutting method.

The glass disk is still completely flat after it is written on by thelaser beam because pits are not formed until the glass isphotographically developed. The disk surface is first made electricallyconductive and then subjected to a nickel evaporation process. The disk,now known as the glass master, then undergoes nickel electrocasting, aprocess which is similar to that used in making analog phono records. Aseries of metal replications follow, resulting in a disk called astamper. The stamper is equivalent to a photographic negative in thesense that it is a reverse of the final compact disk; that is, there arenow bumps were there should be pits. This stamper is then used to make apressing on a transparent polymer such as polyvinyl chloride,poly(ethyl-metacrylate) and polycarbonate. The stamped surface is thenplated with a reflective film such as aluminum or other metal andfinally a plastic coating is applied over the film to form a rigidstructure.

The player operates by focusing a laser beam on the reflective metalthrough the substrate and then detecting reflected light. The opticalproperties of the substrate, such as its thickness and index ofrefraction, are thus critical to the player's detection systems andstandard players are designed specifically with these parameters inmind.

The pits increase the optical path of the laser beam by an amountequivalent to a half wavelength, thereby producing destructiveinterference when combined with other (non-shifted) reflected beams. Thepresence of data thus takes the form of a drop in intensity of thereflected light. The detection system on a standard player is thusdesigned to require greater than 70% reflection when no destructiveinterference occurs and a modulation amplitude greater than 30% whendata is present. These intensity limits, combined with the focusingparameters, set the criteria for the compact disks and other opticaldata storage media which can be read or played on such players.

Media on which data can be recorded directly on and read directly fromhave a different configuration and operate under a somewhat differentprinciple. One example is described in U.S. Pat. No. 4,719,615 (Feyreret. al.).

The medium disclosed in Feyrer et. al, includes a lower expansion layerof a rubbery material which expands when heated. The expansion layer iscoupled to an upper retention layer which is glassy at ambienttemperature and becomes rubbery when heated. Both layers are supportedon a rigid substrate. The expansion and retention layers each containdyes for absorption of light at different wavelengths. Data are recordedby heating the expansion layer by absorption of light from a laser beamat a "record" wavelength to cause the expansion layer to expand awayfrom the substrate and form a protrusion or "bump" extending into theretention layer. While this is occurring, the retention layer rises intemperature above its glass transition temperature so that it can deformto accommodate the bump. The beam is then turned off and the retentionlayer cools quickly to its glassy state before the bump levels out,thereby fixing the bump. Reading or playback of the data is thenachieved by a low intensity "read" beam which is focused on thepartially reflecting interface between the retention layer and air. Whenthe read beam encounters the bump, some of the reflected light isscattered, while other portions of the reflected light destructivelyinterfere with reflected light from non-bump areas. The resulting dropin intensity is registered by the detector. Removal of the bump to erasethe data is achieved by a second laser beam at an "erase" wavelengthwhich is absorbed by the retention layer and not the expansion layer.This beam heats the retention layer along to a rubbery state where itsviscoelastic forces and those of the expansion layer return it to itsoriginal flat configuration. The write, read and erase beams all enterthe medium on the retention layer side, passing through retention layerbefore reaching the expansion layer.

The erasable optical storage medium system described in Feyrer et. al.,has a number of disadvantages. For example, the writing and erasure ofdata must be performed at two different wavelengths of light.

Further, the device relies on reflection at the interface between theretention layer and air which results in an inherently low reflectivity(30% maximum). Thus the system cannot be read by the detection mechanismof a standard compact disk player designed for focusing through a 1.2 mmpolycarbonate substrate and requiring 70% reflectance. Still furtherthere is either a predetermined level of thermal conductivity betweenthe heated expansion layer, to sufficiently raise the temperature of theretention layer so that it can accommodate the bump formed by theexpansion layer, or the retention layer must absorb a predeterminedamount of light energy at the "record" wavelength, in order to producethe needed temperature rise in the retention layer during recording. Ineither case this requirement must be met and accurately controlled ifthis media is to be produced with consistent recording characteristics.In addition, in order for the most effective erasure to be achieved, theretention layer must be heated separately from the expansion layer. Thisfollows from the fact that during erasure the retention layer must reacha rubbery state in order for the visoelastic forces of a cool expansionlayer to pull the expansion layer back to its original flatconfiguration. If the expansion layer is heated during this time, itwill not be in its relaxed state and it will therefore not return to itsflat configuration. Since the expansion layer and the retention layersare in intimate physical contact, heat energy must be conducted betweenthe two layers during both the recordation and erase processes, thusnegating the possibility of only heating the retention layer. Anyattempt to erase the medium during the act of recordation, i.e., directoverwrite data update, would therefore prove unsuccessful.

SUMMARY OF THE INVENTION

A method and apparatus for recording and erasing information on astorage medium is disclosed. The erasable optical storage medium of thepresent invention generally includes a rigid substrate and threeoverlying regions or layers including a first material, a secondreflective material and a third material overlaying the secondreflective layer, the third material optionally covered with a layer ofprotective material. This triple layer formation is susceptible toexpansion and relaxation, to writing data thermally, to erasing datathermally and to reading data optically. Within this formation the firstlayer is optically coupled directly to the substrate as well asphysically bonded to the second reflected layer. The second reflectivelayer is both optically coupled and physically bonded to the first andthird layers. The third layer is optically coupled and physically bondedto the second reflective layer and optionally protected from physicaldamage by a protective fourth layer overlaying the surface opposite thereflective layer-third layer bond. A method and apparatus for recordingand erasing on the medium is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a disk in accordance with the presentinvention.

FIG. 2 is a cross-sectional view of the disk of FIG. 1.

FIG. 3 shows a magnified portion of the disk of FIG. 2 with datarecorded thereon.

FIG. 4 is a block diagram of a system which can be used for recording,reading and erasing data on an optical storage medium in accordance withthe present invention.

FIG. 5 shows an alternative embodiment of the recording medium shown inFIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 generally illustrate the invention described herein as itis applied to a disk 2 which is used to record data in analog or digitalform. The disk 2 includes a first layer 4, a second layer 6, and thirdlayer 8. A substrate 10 and a protective layer 12 may also be provided.FIG. 3 illustrates the invention as shown in FIG. 2 in greater detailwith a recorded "bump".

The first layer 4 is referred to elsewhere herein as the expansion layerand is formed of a material that (a) absorbs a percentage of lightenergy passing through it; (b) displays a high coefficient of thermalexpansion, particularly when compared to the other layers of the medium,and; (c) displays a high coefficient of elasticity to the extent that itwill expand readily when heated at the temperatures encountered during arecordation process without exceeding its upper expansive limit andcontract to its original flat condition upon cooling.

The second layer 6 is referred to elsewhere herein as the centralreflective layer. It is formed of a material that (a) is an alloy,elemental metal, or other reflective material which is soft, malleable,and deformable during the course of the recordation or erasure process,and; (b) reflects sufficient light energy such that the total lightenergy loss sustained by a monochromatic beam of light or otherradiation (which is incident upon the substrate 10, and travels throughthe substrate, then through the first material layer 4, reflects off ofthe second layer 6, traverses again through the first material layer andthen passes through the substrate once again and out of the media) isless than 30%. In an alternative embodiment, the second layer is amaterial with insulating properties or insulating and reflectiveproperties.

The third layer 8 is referred to elsewhere herein as the retentionlayer. It is formed of a material that (a) absorbs a percentage of lightenergy passing through it; (b) displays a glass transition temperaturewhich is above room temperature; (c) is rubbery, when above its glasstransition temperature, with sufficient elasticity to permit it toconform to the contour of the distortion formed in the second layer bythe expansion of the first layer, when the first active material layeris heated, and; (d) displays sufficient rigidity and strength below itsglass transition temperature such that it will hold the second and firstlayers in the stretched, expanded condition which was effectuated whilethe first active layer was heated, even though the first layer hascooled to ambient temperature. It should be noted that in someembodiments, only the third layer remains in a deformed condition, andthe first layer returns to its undeformed state. Data are then read fromthe retention layer.

Writing is achieved in one embodiment by focusing two cooperatingmonochromatic light beams, chosen from a wide spectrum of availablelight wavelengths, onto the central reflective second layer 6 fromopposite sides of the media, and modulating these beams with data. Theangle of incidence of both these beams is preferably adjusted to about90 degrees (e.g., 90 degrees ±1 degree) in order to assure that thebeams create a circular cylinder of energy within the active media. Thefirst of these beams will pass through the substrate 10 and expansionlayer 4, be reflected off the central reflective layer 6, pass throughthe expansion layer once again and exit through the substrate. Thesecond of these beams will pass through an optional protective layer 12,and the retention layer 8, be reflected off the central reflectivelayer, pass through the retention layer once again and exit through theoptional protective layer, if present.

The first beam, referred to herein as the record beam and indicated inFIG. 2 with arrow 14, causes the heating and subsequent expansion of theexpansion layer 4 in the direction away from the substrate 10, therebyforming a bump, as illustrated by reference numeral 16 in FIG. 3 in thedirection of the reflective layer. The reflective layer reverses thedirection of this modulated incident beam and returns the incidentbeam's light energy directly back along the same path from which itcame, thereby heating the expansion layer on both entrance and exit.Because the reflective layer does not display 100% reflectance, aportion of the energy incident upon the reflective layer will heat thisreflective layer above ambient temperature. The material comprising thereflective layer is chosen to be malleable and deformable during thecourse of the recordation process. Therefore, the bumps formed by themodulated record beam in the expansion layer will protrude into the softreflective layer and the reflective layer will conform around thesebumps.

The data used to modulate the record beam are binary, digital, i.e., thedata are composed of either ones or zeros, the presence of a bump edgeindicating a one and the absence of a bump edge representing a zero.These "digital bits" are presented to the record beam at a preselectedrate. Therefore it is known beforehand when a zero or one can bewritten. A synchronization signal can be used which is composed of veryshort pulses that indicate the start of the period of time during whicha bump can be written. These short pulses themselves occur at the samepreselected and constant rate as employed by the data's "digital bitstream". In the present invention, the synchronization signal is alsoused to modulate the second beam, known as the erase beam.

The modulated second beam, known as the erase beam (indicated with arrow18), heats the retention layer 8 in synchronism with the data presentedto the record beam. This erase beam is turned on at the start of the"bit period" and turned off before the end of the "bit period"; the timethe first beam is first turned on for each "bit period" being dictatedby the synchronization signal described above. This assures that theretention layer is above its glass transition temperature when therecord beam is writing each individual data bit on the media.

The retention layer material is therefore soft during the bit recordingprocess and allows the expansion layer bumps, overlayed with reflectivematerial from the reflective layer, to protrude into its volume. Beforethe record beam completes the writing of any individual data bit, theerase beam turns off. This causes the retention layer to cool whileenergy is still being applied to the bump forming process taking placein the expansion layer. By this mechanism the cooled retention layer,which has reached its glass transition temperature, holds the reflectivematerial covered bumps into position after the expansion layer hascooled.

Since the above described method does not require the use of twodifferent or specific wavelengths of light energy for separate heatingof the retention and expansion layers, any monochromatic beams of lightthat are absorbed by these layers, can be employed for the write/erasureprocess. Further, since (a) light energy is injected into the expansionand retention layers separately in a physical as well as a timing sense;(b) the reflection layer acts to isolate the transfer of energy betweenthese two layers; and (c) the reflection layer also acts to cause eachlight beam to double pass through each active layer separately, withoutlosses from any other intervening layers of material, more light energycan impinge on each of these layers during the write/erase processwithout causing deleterious thermal interactions between the activelayers. Therefore the novel media being disclosed can be constructedsuch that it will display lower light energy absorption as compared tothe prior art.

Still further, since the described media does not require the use ofspecific wavelengths of light, single dyes or combinations of dyes whichabsorb energies over all wavelengths, preferably excluding the readwavelength, can be employed to increase the sensitivity of the media tothe heating effects of the recording and erase beams.

The amount of power that is injected into the expansion and retentionlayers may be separately variable by changing the record and erasebeams' pulse width and height. This allows closer control of bumpformation and retention during the recording and erasure process as wellas the opportunity to use a wider range of material as the basis for thethree active layers of the unique triple layer media being disclosed.

The new triple media in conjunction with the dual beam cooperativerecording mechanism previously described has the ability to directlyerase and write on media which has been previously recorded on withoutthe need to erase this media first. In this mode, when the erase beamsoftens the retention layer, such that it is above its glass transitiontemperature, it will release any bumps previously imbedded in theretention layer and allow the expansion layer to relax at theselocalized points. If the recording beam is writing a "zero" at thistime, i.e., the beam is off, the localized point involved will relax orremain relaxed regardless if it had a "one", i.e., a bump, written on itbefore or had a "zero", i.e., was already in its relaxed state, writtenon it before. If the recording beam is on, the localized point involvedwill expand or remain expanded regardless if it had a "zero", i.e., wasin its relaxed state, before or had a "one", i.e., a bump written on itbefore.

Reading is achieved by focusing a monochromatic light beam, chosen froma wide spectrum of available light wavelengths, through the substrate10, through the first material layer 4, and onto the reflective layer 6.Taking into consideration the losses through the retention layer andsubstrate, this reflective surface provides the percentage of reflectionnecessary to allow reading of the bumps (data) by a standard compactdisk player read mechanism.

Erasing is achieved by focusing a monochromatic light beam, chosen froma wide spectrum of available light wavelengths, through the optionalprotective layer, if present, through the third material layer 8 andonto the central reflective layer. This erase beam heats the retentionlayer above its glass transition temperature and thereby allows anybumps previously imbedded in the retention layer to be released. Sincethe expansion layer is not being heated at this time and is cool,contraction forces in this layer will locally pull the expansion layerinto a flat configuration and the bumps (data) will disappear.

The expansion layer is formed of a material or combinations of materialswhich display some light absorption at the wavelength of the recordbeam. The wavelength of this beam of light may be chosen from a widespectrum of available light wavelengths. The degree of absorptivity mayvary from wavelength to wavelength and from expansion material toexpansion material. For example, however, this degree of absorptivitycould be between 20% and 40% in the wavelength range from 850 nm to 650nm. To maintain the ability to read data recorded on this unique opticalmedia on standard detection mechanisms, such as those found onconventional compact disk players, a maximum double pass absorption atthe compact disk read wavelength (780 nm) of 10% is preferred. Inaddition, unlike prior art, there is no fundamental reason whichrequires that the recordation wavelength be different from the erasurewavelength. The recordation wavelength chosen can and is preferred tobe, the same wavelength as used for erasure.

The expansion layer has a high coefficient of thermal expansion,particularly when compared to the other layers of the medium.Coefficients above about 1×10-4/° C. preferred, with those greater thanabout 5×10-4/° C. particularly preferred and those greater than about7.5×10-4/° C. most preferred.

In addition, the expansion layer material is rubbery at ambienttemperature, i.e., having a high coefficient of elasticity, to theextent that it will expand readily during recordation without exceedingits upper expansive limit. When at room temperature, the expansion layermaterial is near or above its glass transition temperature, which ispreferably below 30° C.

The reflective layer 6 serves to reflect light (e.g. more than 25% ofthe light striking it) back through the expansion layer 4 for thepurposes of improved data recordation and data detection. During therecordation process the reflective properties of the reflective layercauses the recording light beam to double pass through the expansionlayer, thus doubling the effective light beam path inside the expansionlayer. Energy for the purposes of heating and thus expanding theexpansion layer is thereby absorbed for both directions of therecordation light beam.

The reflective layer also serves to thermally and optically isolate theexpansion layer from the retention layer, which overlays the reflectivelayer. Separate laser light source acting in cooperation, can beemployed for the reading/erasure process. Alternatively, the same sourcecould be used by employing, for example, a beam splitter.

So that recorded data is able to be read by standard compact disk playerreading mechanisms, the reflective layer provides the means forreflecting light energy incident on the media's substrate back throughthe media's substrate, after this light energy has been modulated byrecorded data. Since the reflective layer is deformable it conforms tothe shape of the deformations in the expansion layer, which representthe recorded data. Therefore, an incident read light beam is effectivelymodulated by quarter wavelength interference as well as lightscattering.

The retention layer 8 is formed of material or combinations of materialswhich display at least some light absorption at the wavelength of theerase beam. The wavelength of the erase beam 18 light may be chosen froma wide spectrum of available light wavelengths. The degree ofabsorptivity may vary from wavelength to wavelength and from retentionmaterial to retention material but may be for example about 30% to 45%at wavelengths between 650 nm and 860 nm. To facilitate the ability toaccurately erase data recorded on this unique optical media it isdesirable to be able to read data through the retention layer. Thereforeit is preferable to limit the maximum double pass absorption of theretention layer at the erase wavelength to less than 80% even though thesystem can be made to work with more absorption. In addition, unlikeprior art, there is no fundamental reason which requires that theerasure wavelength be different from the recordation wavelength. Theerasure wavelength chosen can and is preferred to be, the samewavelength as used for recordation.

The retention layer material has a glass transition temperature which isabove ambient temperature and very much above the glass transitiontemperature exhibited by the expansion layer. In general, this glasstransition temperature will range from about 50° C. to 300° C.,preferably it will lie between 75° C. to 125° C. When above the glasstransition temperature, the material is rubbery with a sufficientelasticity to permit it to deform to the contour of the distortionformed in the reflective layer caused by the expansion of the expansionlayer without exceeding its elasticity limit.

In further embodiments of the invention, the retention layer has a highthermal conductivity, causing it to rapidly rise in temperature aboveits glass transition temperature when light energy from the erase beamis applied. After the erase beam has been turned off, this high thermalconductivity serves to foster the rapid cooling of the retention layerto its glassy state. Since, at this time, light energy is still beingapplied to the expansion layer by the record beam, the expansion layeris still in its enlarged condition and the reflection layer is stilllocally deformed by the expanded expansion layer. The cooled retentionlayer can now hold, because of tight bonding between the reflection andexpansion layers, the deformed reflective and expansion layers in theirextended positions after the write beam is no longer exciting theexpansion layer and the expansion layer has cooled. A retention layerhaving a thermal conductivity of at least about 2.5×10⁻⁴ cal/((cm² /°C.)(sec/cm)) will provide adequate results.

Certain embodiments of the present invention also include a protectivelayer 12 placed over the retention layer 8 to protect it from damage dueto contact with external objects. Characteristics of the protectivelayer are:

(A) low absorption of light energy at all wavelengths, but particularlyat the erase wavelength.

(B) sufficiently compliant to allow the deformations in the reflectivelayer to easily protrude into it and thereby offer little resistance totheir formation.

(C) sufficient thickness such that the bumps formed in the reflectivelayer are not transmitted through the retention layer, into theprotective layer and then subsequently through the protective layer tothe outer surface.

(D) high thermal conductivity to enable it to serve as a heat sink forpurposes of rapid cooling of the retention layer, immediately after theerase beam impinging on its surface is turned off. A thermalconductivity of at least 5×10-4 cal/((cm² /°C.) (sec/cm)) will provideadequate results. The protective layer is preferably 2×10⁻³ cal/((cm²/°C.) (sec/cm)).

The various layers described above are arranged on a substrate with theexpansion layer which may be directly bonded to the overlayingsubstrate, the reflective layer bonded to and directly overlaying theexpansion layer, the retention layer bonded to and directly overlayingthe reflective layer and the optional protective layer bonded to anddirectly overlaying the retention layer. The substrate itself is formedof a rigid transparent material which permits substantially fulltransmission of light at all wavelengths. The substrate is sufficientlythick and rigid to provide structural integrity to the medium and doesnot deform in response to pressure caused by expansive forces in theexpansion layer. Bulges in the expansion layer, caused by its thermalexpansion upon absorption of the record beam's light energy, protrudeaway from the substrate due to the substrate's rigidity. With this layerarrangement, the bulges protrude into the reflective and retentionlayers, causing their deformation as described above.

The thickness of the layers will be selected in accordance with theoptics of the system. For example, in order to maintain the minimum marksize during data recordation with the greatest write sensitivity duringrecording, the laser beam should be maintained as small as possible asit passes through the expansion layer. Accordingly, most of theexpansion layer 4 should be within the focal depth of the record beam.For recording systems having optical parameters similar to those foundin standard compact disk players, the record beam is diffraction limitedand has a focal depth of approximately 1.0-2.0 microns. In such cases,best results can be obtained with an expansion layer preferably 1.0microns or less.

In a similar manner, to maintain the minimum erasure area during thecourse of data erasure, the laser beam should be maintained as small aspossible as it passes through the retention layer. Accordingly, most ofthe retention layer 8 should be within the focal depth of the erasebeam. For erasure systems having optical parameters similar to thosefound in standard compact disk players, the erase beam is alsodiffraction limited and has a focal depth of approximately 1.0-2.0microns. In such cases, best results can be obtained with a retentionlayer having a thickness similar to that of the expansion layer orapproximately 0.5 to 1.5 microns, preferably 1.0 microns or less.

The substrate and the optional protective layers are considerablythicker, the substrate layer being on the order of 1 millimeter or moreand the optional protective layer being on the order of tens of microns,in view of their respective functions (i.e., the substrate must be thickenough to impart rigidity to the medium and the protective layer must bethick enough to protect the data protrusions from external abuse). Thesubstrate is preferably 1.2 mm. The protective layer is preferably 2 μm.

The materials used in forming the layers will be selected on the basisof the properties indicated above, i.e., transparency, reflectivity,absorptivity, glass transition temperature, elasticity and thermalexpansivity. The preferred materials for all layers except thereflective layer are amorphous polymers. Examples of such materials arerubbers, natural rubbers such as butyl rubbers, silicone rubbers,natural rubbers and styrene-butadiene rubbers; polymers such ascellulose acetate, cellulose acetate- butyrate, polystyrene,polysulfonamide, polycarbonate, cellulose nitrate,poly(ethyl-methacrylate), poly(vinyl butyryl), aromatic polyesters,polyamides, acrylic polymers, polyvinyl acetate, silicone resins, alkydresins, styrene- butadiene copolymers, vinyl chloride-vinyl acetatecopolymers, nitrocellulose, ethylcellulose and polyvinyl alcohol; andsubstances such as gelatine glue, casein, egg albumin and dihydroabietylalcohol. Materials with high elasticity such as elastomers and polymerswith elongations greater than 15% are preferred for construction of theexpansion layer. Materials with relatively high glass transitiontemperatures, notably greater than 50° C., with elongations greater than5%, such as those found in the epoxy family of resins, are preferred forconstruction of the retention layer.

The reflective layer may be formed of any reflective material which issufficiently elastic and malleable to conform to the bulges protrudingfrom the expansion layer. The material should not unduly constrict bumpformation and should not become substantially work hardened over thenumber of desired write and erase cycles. Examples of such materials aregallium, aluminum, copper, silver, gold and indium. Other examples arealloys, particularly eutectic alloys of bismuth with tin or cadmium.

The absorptive characteristics of the various layers may be impartedthereto using methods that will be apparent from the above disclosureand known to those of skill in the art. Since the media of the presentinvention need not be wavelength specific, a broad range of dyes orpigments are available for this purpose. In addition, except for theability to pass a portion of the wavelength energy which is employed forthe purpose of reading the recorded data, these dyes or pigments neednot be wavelength specific and may therefore absorb light energy over abroad spectrum of wavelengths. Since standard compact disk playerdetection mechanisms require a minimum of 70% reflection, as seen bylight which is incident on the substrate side of the media, and employlaser diodes which function at 780 nm wavelength for the purpose ofreading data, it is preferable to limit the maximum double passabsorption of the expansion layer at this wavelength to less than 10%even though the system can be made to work with more absorption. Dyes orpigments which may be used singly or in combination are nigrosin blue,aniline blue, Calco Oil Blue, ultramarine blue, methylene blue chloride,Monastral Blue, Malachite Green Ozalate, Sudan Black BM, Tricon blue,Macrolex green G, DDCI-4 and IR26.

The various layers of the media of the present invention are bondedtogether according to conventional techniques. It is preferred thatadjacent layers be optically coupled to one another such thatsubstantially all light propagated through one layer enters, or isreflected off, the adjacent layer.

The media of the present invention may be fabricated in accordance withconventional techniques, whereby the various layers are applied insuccession by coating over a substrate. Knife spread techniques, spincoating techniques and metal vapor deposition are examples of techniqueswhich may be used.

A second alternative embodiment of the media discussed above is shown inFIG. 5. This embodiment includes a first layer 102, a second layer 104,a third layer 106, and a further layer 108. A substrate and protectivelayer (not shown) may also be provided. The first layer 102 is theexpansion layer, as described above. The second layer 104 is atransparent, malleable insulation layer. The third layer 106 is aretention layer and the fourth layer 108 is a malleable reflectivelayer. The first and third layers are dye loaded such that the firstlayer, the expansion layer, will absorb light energy at, for example,680 nm but not at 830 nm and the third layer, the retention layer willabsorb light energy at 830 nm and not at 680 nm. The second, insulatinglayer allows separate heating of the first and third layers by the useof lasers that emit light at 680 and 830 nm. Therefore data stored onthe media can be directly overwritten. In fact by using cooperating andsynchronized write and erase beams as described below all of the samebenefits are achieved, except for the ability to use the same wavelengthof light for both record and erase. The second layer is preferably softand malleable and can not hold the first layer in a stretched, expandedstate.

FIG. 4, illustrates one embodiment of an electro-optical system forrecording data on the storage media in accordance with the presentinvention and includes three separate laser beam sources, one forwriting, one for erasure and one for reading. In this scheme three lasersources are included in order to provide direct overwrite capability.Write beam diode 20 and read beam diode 22 are both directed at a first,highly polarizing beam splitter 24. The write beam diode 20 is rotatedso that the merging write beam 26 is S-polarized with respect to thebeam splitter 24, while the read beam 28 is P-polarized, with respect tothe beam splitter 24, by 1/4 wavelength plate 30. Beam splitter 24 isitself arranged so that it reflects a high proportion of the write beam26 downward along, for example, the Y axis while transmitting a highproportion of the read beam 28, with the result that the reflected writebeam and the transmitted read beam are offset but parallel, forming twospatially separated beams 32 and 34 respectively toward the recordingmedium 2. The highly polarized beam splitter 24 preferably reflects atleast 90% of write beam 26 and transmits at least 90% of read beam 28,with about 95% to 98% preferred for both polarization directions.

A second beam splitter 36, less polarizing than the first, is situatedin the path of two parallel beams 37 emerging from the first beamsplitter. The second beam splitter 36 is rotated 90 degrees with respectto the first beam splitter 24, with the result that any light reflectedfrom the Y axis is directed along the Z axis. Also due to the change ofdirection, the second beam splitter 36 causes reflection of light fromthe read beam 28 transmitted through the first beam splitter 24. Sincethe second beam splitter 36 is less polarized than the first, however,it transmits preferably from about 10% to about 50% of the light fromthe read beam, reflecting substantially the remainder. In a convenientarrangement, 80% of the read beam is reflected along the Z axis to loss38 and 20% is transmitted along the Y axis toward the recording medium.For light polarized in accordance with the write beam 26, however, it ispreferred that at least about 80% be transmitted down along the Y axistoward the recording medium, with the rest reflected to loss 38,exclusive of the amount absorbed. Convenient values for the write beamare 88% transmission and 10% reflection.

A collimating lens 40 and an objective lens 42 focus the write and readbeams on the recording medium 2, particularly the reflecting surfacetherein. The reflecting surface reflects light less any light which hasbeen absorbed by the medium, back along the same axes, superimposed overthe incident beams 32 and 34, toward the second beam splitter 36. Thereflected read beam 34, due to its polarization, is partially reflectedand partially transmitted as before, with the same percentages. Thereflected beam 44, however, is directed along the Z axis toward thephotodetector 48. In the specific example given above, 80% of the readbeam reflected from the recording medium will thus be directed towardthe photodetector. The reflected write beam will also be present. Themajority of this beam will be transmitted through the beam splitter 36as before, with a small reflected portion (about 10% in the specificexample) being reflected toward the photodetector.

The write beam 26 will be of a sufficient intensity to cause a recordingresponse in the medium. The read beam 28 will generally be of lesserintensity. Specifically, power of the write diode 20 and the read diode22 are expected to be about 30 mW and 5 mW respectively. An advantage ofthis arrangement is that a relatively small portion of the write beamreaches the photodetector 48. This proportion may be reduced evenfurther by choosing a read beam wavelength which is different than thatof the write beam and separating these two beams by use of appropriatefilters. By proper wavelength selection of both the write and read beamit is possible to implement an embodiment of the current invention whichdoes not require the use of polarizing elements to sufficiently separatethe write and read beams during the recordation process.

The two beams, write beam 32 and read beam 34, preferably impinge uponthe medium 2 such that the focal point of the read beam scans the mediasurface slightly ahead of the focal point of the write beam. The exactdistance that the focal point of read beam 34 is ahead of write beam 32is not critical for the media 2 is being rotated at a constant linearvelocity. Therefore by knowing the distance between read beam 34 andwrite beam 32, the time at which a bump edge, detected by the read beam,will appear beneath the write beam can be easily and preciselydetermined. For best results, however, read beam 34 should be separatedfrom write beam 32 by at least 0.5 microns, to assure that they do notoverlap, with a total separation of less than 2 microns, to allowsufficient focusing action by a single set of optics.

The data derived from read beam 34 are detected by photodetector 48 andsignal receiver 50. After processing by signal receiver 50 they are sentto digital signal processor 52 along line 54, where the data areemployed for proper synchronization of the drive write beam 26 and erasebeam 56. A processed version of the read data is also sent on line 58 towriting and reading focus and tracking servo unit 60 and, throughinverting amplifier 62, to erase beam focus and tracking servo unit 64.These two units, 60 and 64, employ the read data for the purpose ofcausing write beam 32, read beam 34 and erase beam 56 to be in focus andthe proper position for performance of recordation, reading and erasureof data. It should be recognized that reflections of the erase beamcould be monitored in a manner similar to that described above and,therefore, the read beam could be eliminated in certain embodiments.

The reflected read laser beam 44 from disk 2 is sent through cylindricallens 66 to photodetector 48. This lens causes the beam to gradually varyits shape along its path, first as an oval in a longitudinal direction,then as a circle, and finally as an oval in the transverse direction.This beam shape varies as function of the distance the read beam's focalpoint is away from the reflective surface of the media. The photodiodesin photodetector 48 are divided in four sections, all of which provideoutputs when read beam 45 is in focus. However, when the disk is tooclose, the longitudinal direction beam supplies a light signal to theupper and lower photodiodes, causing only these two diodes to provideoutputs. On the other hand, if the disk is too far, only the left andright diodes provide an output. By amplifying the difference in outputderived from these four diodes, a focus error signal is obtained. Thisfocus error is amplified and placed on line 68. This line splits anddrives write beam and read beam focus and tracking servo 60 directly anderase beam focus and tracking servo 64 through inverting amplifier 70such that write beam 32, read beam 34 and erase beam 56 remain in focus.

A heterodyne method is one approach that can be employed by the presentinvention for the purposes of correcting for tracking error. This methodinvolves monitoring whether or not read laser beam 41 is applied equallyto four division photodiode detector 48. The photodetector assembly is acircular array of photodiodes divided in four quadrants (A through D). Ais the upper left hand quadrant, B is the upper right hand quadrant, Cis the lower left hand quadrant and D is the lower right hand quadrant.When the beam is located at the center of the track, the output of(A+C)-(B+D) becomes zero. However, if the beam is deflected from thecenter of the tract the waveform of (A+C)-(B+D) changes based on thedirection and extent of the deflection. This signal is placed on line58. This line splits and directly drives write beam and read beam focusand tracking servo 60 and erase beam focus and tracking servo 64,through inverting amplifier 62, such that read beam 34, write beam 32and erase beam 56 stay "on track".

Note that erase beam 56 should be properly focused and track the samepoint as being addressed by write beam 32. The characteristics of eraseservo 64 are adjusted to exactly match the characteristics of write/readservo 60 and that movement of the disk toward the read beam is alwaysmatched by an equal an opposite move away from the erase beam.Therefore, one way of correcting for erase focus, as shown in thisembodiment, is to first focus the erase beam manually and thereaftercorrect for focus by inverting the focus control signal appearing online 68, using inverting amplifier 70, and employing this signal todrive erase servo 64 through line 72. The same concept if used in thisembodiment for assuring that erase beam 56 always is on the same trackas write beam 32. In this case a movement of a track toward the left asviewed from the top of the disk appears as a movement toward the rightas viewed from underneath the disk. Therefore, the erase beam is firstaligned with the record beam manually and thereafter corrected foralignment by inverting the tracking control signal appearing on line 58,using inverting amplifier 62, and employing this signal to drive eraseservo 64 through line 74.

There are other means of assuring that erase beam 56 is properly focusedand tracks the exact same point as being addressed by record beam 32. Byadding the elements equivalent to read laser 22, 1/4 wavelength plate30, beam splitter 24, second beam splitter 36, cylindrical lens 66,photodetector 48, and signal receiver 50 to the erase beam optical path,erase beam 56 would be capable of tracking and focusing independent ofread beam 34. No manual beam alignment or focusing set up procedurewould be required. Additionally, by making the "erase-read" beam coaxialwith erase beam 56, these added components would permit reading datastored, and currently being altered, on the media by the action ofrecord beam 32. Comparing signal 78, which drives the record laser, withthe signals detected by the "erase-read" photodetector, allows simpleand reliable identification of the track currently being overwritten.This information would be employed to drive erase beam focus andtracking servo 64 so that erase beam 56 always addressed the same pointbeing addressed by record beam 32.

In the preferred embodiment, the write 34 beam and erase laser beam fromthe erase laser 79 are digitally modulated in accordance with a versionof the data appearing on line 80. However, a workable mechanism forrecording, reading and erasing data on a storage media constructed inaccordance with the present invention, can be derived by employing anerase beam which is continuous and not modulated. In this embodiment,the erase beam's light energy would be carefully chosen to providesoftening of the retention layer during the recordation process, butstill allow the retention layer to cool to its glass transitiontemperature, as previously described, before the expansion layer hadcooled to its relaxed condition. It should be noted that when thisprocess is employed, it is not necessary for the continuous erase beamto be focused on the exact same spot as that focused upon by the writebeam. In fact, by having the continuous erase beam focus on a spot whichslightly precedes the spot focused upon by the write beam, an additionalmeasure of erasure control is provided. The erase beam would thenpreferably precede the write beam by about 2 μm.

The preferred embodiment utilizes a processed version of the dataappearing on line 80, which is synchronized with the write laser drivesignal appearing on line 78 to drive the erase laser 56. Digitalprocessor 52 in conjunction with data read by read beam 34, appearing online 54, creates a modulated erase beam which heats the media'sretention layer in synchronism with the data to be recorded. Signalprocessor 52 could, for example, be a ADSP2100 manufactured by AnalogDevices. Erase beam 56 is turned on at the start of the record "bitperiod" and turned off before the end of the record "bit period". Theretention layer 8 of FIG. 3 is therefore soft during the bit recordingprocess and allows the expansion layer bumps 16, overlaid withreflective material from the reflective layer to easily protrude intovolume of the retention layer 8. It also assures that retention layer 8begins cooling toward its glass transition temperature while theexpansion layer 4 is still being heated by record beam 32. By thismechanism the retention layer will reach its glass transitiontemperature before expansion layer 4 relaxes to its flat state, holdingthe reflective material covered bumps into position after the expansionlayer has cooled.

It is necessary to synchronize the data appearing on record beam 32 anderase beam 56 with any data that may be already recorded on the media.This is done to assure that the write/erase action occurs only at thebump transitions that have already been formed in the media. If thissynchronizing action is not performed, a mixture of the two data streamswould be recorded on the disk, resulting in an inability to recover boththe previously and newly recorded data. Given the synchronizationmechanism described above, the scheme being presented provides fordirect overwrite recording. If a bit formed in previously recorded mediapasses between the record/erase beams of the present invention, it willbe removed or remain in place depending upon whether or not the recordbeam is attempting to record a data bit at that location. If a data bitis being recorded, the energy from record beam 32 will force expansionlayer 4 of FIG. 2 to remain expanded at that point after the erase beamis no longer active. If no data bit is to be recorded at that point, theexpansion layer will not be forced to remain in an expanded conditionbecause there will be no energy available from write laser 9. Erase beam56 will soften retention layer 8 and release the expansion layer,allowing it to relax and assume a flat condition. Therefore the presentinvention effectively erases data bumps during the recordation processand provides for direct overwrite capability.

If a "virgin" disk, i.e., one which has never been recorded on, isutilized, signal processor 52 would receive no intelligible data on line54 from signal receiver 50 and read photodetector 48 and fall into aself synchronization mode to effect proper recording.

The process of erasure separate from recording is the same as theprocess of recordation described above except that no signal is placedon line 78. Thus write driver 80 and write laser 20 are inactive. Thisresults in a single erase beam impinging on the disk which issynchronized with the data already appearing on the disk. Since there isno write beam to cause expansion layer to expand, the heated retentionlayer 4 will soften and release any data bumps 16, which will allow themto assume their relaxed, flat condition. A non-modulated,non-synchronized erase beam can also be employed for the same purpose.

The signal receiver, the signal processor, the write driver, and theinverting amplifiers are of the type readily known to one skilled in theart. Table 1 lists one of a wide variety of commercially availablecomponents that could be used in the invention described herein.

                  TABLE 1    ______________________________________    COMPONENT INFORMATION    Component       Model and Manufacturer    ______________________________________    Write diode 20  Model No. SDL-5410                    Manufactured by Spectra Diode    Read diode 22   Same as write diode    Beam Splitter 24                    From Olympus TAOHS-LC3    1/4 wavelength plate 30                    From Olympus TAOHS-LC3    Beam splitter 36                    From Olympus TAOHS-LC3    Photodetector 48                    From Olympus TAOHS-LC3    Signal receiver 50                    Rocky Mountain Z Channel Servo                    Amplifier    Signal processor 52                    Analog Devices ADCP-2100    Servo 60        From Olympus TAOHS-LC3    Inverting amplifier 62                    Teledyne Philbrick TPOO32    Servo 64        From Olympus TAOHS-LC3    Inverting amplifier 70                    Teledyne Philbrick TPOO32    Driver 82       Meleos Griot Diode Laser Driver                    06 DLD001    ______________________________________

In alternative embodiments of the invention a storage medium is mountedon a rigid substrate in which optical data can be recorded through thesubstrate by a record light beam and from which data so recorded can beread by a read light beam reflected back through the substrate into adetection system designed to read signals reflected through a layerwhich is substantially the optical equivalent of said rigid substrate.The storage medium may comprise an expansion layer optically coupled tothe substrate along an interface therebetween and expandable uponheating to form protrusions on the surface thereof opposite theinterface; means for retaining protrusions so formed in said expansionlayer upon cooling thereof; and a reflective layer which reflects atleast about 25% of the light striking it and is sufficiently elastic toconform to the contour of said surface. The retaining means may be aretention layer comprised of material which has a glass transitiontemperature substantially above ambient temperature and thereby convertsfrom a glassy state at ambient temperature to a rubbery state at atemperature above said glass transition temperature and the retentionlayer may be coupled directly to at least one of said expansion layerand said reflective layer while the retention layer may be between theexpansion layer and the reflective layer and coupled directly to each.The reflective layer may reflect at least about 85% of the light fromsaid read light beam passing through the expansion layer back throughthe expansion layer and the substrate.

Alternatively, the storage medium may be mounted on a rigid substrate inwhich optical data can be recorded through the substrate by a recordlight beam and from which data so recorded can be read by a read lightbeam reflected back through the substrate into a detection systemdesigned to read signals reflected through a layer which issubstantially the optical equivalent of the rigid substrate, the storagemedium comprising an expansion layer of approximately 0.5 to 1.5 micronsin thickness optically coupled to the substrate along an interfacetherebetween and expandable upon heating to a temperature substantiallyabove ambient temperature to form protrusions in the surface thereofopposite the interface; a retention layer of approximately 0.25 to 1.0micron in thickness optically coupled to the expansion layer andcomprised of a material which has a glass transition temperaturesubstantially above ambient temperature and thereby converts from aglassy state at ambient temperature to a rubbery state at a temperatureabove said glass transition temperature; a reflective layer coupled tothe retention layer and which reflects at least about 85% of the lightfrom the read light beam passing through the expansion and retentionlayers back through the expansion and retention layers and thesubstrate, and is sufficiently elastic to conform substantially to thecontour of said retention layer; and a protective layer on the sidethereof toward which the protrusions extend, the protective layer beingof material sufficiently deformable to receive the protrusions; thethicknesses of said expansion layer and the retention layer each beingabout 1/1000th or less of the thicknesses of the substrate and theprotective layer, and the retention layer having a thickness of about1.0 micron or less. The record light beam and the read light beams mayeach be substantially monochromatic at first and second wavelengthsrespectively and the reflective layer may reflect at least about 85% oflight of the first wavelength and the second wavelength incidentthereupon.

The expansion layer may absorb at least about 40% of light at the firstwavelength passing and reflected back therethrough or less than about90% of light at the first wavelength passing and reflected backtherethrough. Alternatively, the expansion layer absorbs at least about50% of light at the first wavelength passing and reflected backtherethrough, and transmits at least about 60% of light at the secondwavelength passing and reflected back therethrough. In still anotherembodiment, the expansion layer absorbs from about 50% to about 85% oflight at said first wavelength incident upon it and transmits at leastabout 80% of the light of said second wavelength incident thereupon.

An elongate region defined as a track may be designated for storage ofoptical data, and portions of the interface along a segment of the trackmay be displaced to protrude into the substrate to form a binarysequence of height variations in the interface along the track. In oneembodiment, the portions are of a height ranging from about 0.1 to about3.0 times the thickness of the expansion layer or, alternatively, fromabout 0.5 to about 2.0 times the thickness of said expansion layer. Aplurility of the binary sequences may be positioned at regular intervalsalong said track. The length of the track may be comprised of a seriesof sublengths each corresponding to one frame on a standard compact diskand each having a leading end and tailing end, and one of thepreselected binary sequences is located at the leading end of eachsublength. A plurality of the binary sequences may be positioned alongthe track, the binary sequences representing coded instructions tosynchronize the movement of the track with a detection system. Aplurality of the binary sequences positioned along the track, the binarysequences representing coded information regarding the distance of thebinary sequences along the length of the track. The binary sequencespositioned along the track, may represent a coded directory of data tobe recorded on the track.

A method for recording data on an optical data storage medium mounted ona rigid substrate may comprise (a) passing a beam of light at apreselected wavelength through the substrate; (b) passing light emergingfrom the substrate through an expansion layer optically coupled theretowhich is partially absorptive of light at the preselected wavelength andwhich expands with increasing temperature; (c) reflecting lighttransmitted through the expansion layer off of a deformable reflectinglayer and back through the expansion layer, to expand the expansionlayer with heat generated by light absorbed thereby combined with lightabsorbed in step (b), and thereby form a protrusion on the surfacethereof facing away from said substrate and a deformation in saidreflecting surface conforming to the protrusion; (d) heating byconduction from the expansion layer a retention layer coupled to thereflecting layer, the retention layer having a glass transitiontemperature substantially above ambient temperature, to raise thetemperature of the retention layer above the glass transitiontemperature to permit the protrusion to deform the retention layerthereto; and (e) cooling the retention layer below the glass transitiontemperature while so deformed to fix said protrusion in the expansionlayer. The retention layer may be optically coupled to the expansionlayer and may be between the expansion layer and the reflective layer.The retention layer may be partially absorptive of light of thepreselected wavelength incident thereupon. Step (e) may be achieved inpart by diffusing energy from the retention layer into a heat absorptivelayer which dissipates heat at a rate faster than the expansion layer.The expansion layer may absorb at least about 40% of light at thepreselected wavelength incident thereupon. The expansion layer mayabsorb less than about 90% of light at the preselected wavelengthincident thereupon or, alternatively, from about 50% to about 85% oflight at the preselected wavelength incident thereupon. The preselectedwavelength may be defined as a first preselected wavelength and theretention layer may absorb light at a second preselected wavelength towhich the expansion layer is substantially transparent.

It is to be clearly understood that the above description is intended tobe illustrative and not restrictive. For example, while the descriptionhas been written with regard to the use of lasers as the radiant energysource, other energy sources could be used and would be readily apparentto those of skill in the art. Further, it would be possible to reversethe roles of the erase beam and write beam from that described above,i.e., the write beam could be directed at the medium substantiallycontinuously and the erase beam could be modulated in response to datato be recorded. The scope of the invention should, therefore, bedetermined not with reference to the above description, but shouldinstead be determined with reference to the appended claims, along withthe full scope of equivalents to which they are entitled.

What is claimed is:
 1. A method for recording data on an optical datastorage medium mounted on a rigid substrate, comprising:(a) passing abeam of light at a preselected wavelength through said substrate; (b)passing light emerging from said substrate through an expansion layeroptically coupled thereto which is partially absorptive of light at saidpreselected wavelength and which expands with increasing temperature;(c) reflecting light transmitted through said expansion layer off of adeformable reflecting layer and back through said expansion layer, toexpand said expansion layer with heat generated by light absorbedthereby combined with light absorbed in step (b), and thereby form aprotrusion on the surface thereof facing away from said substrate and adeformation in said reflecting surface conforming to said protrusion;(d) heating by conduction from said expansion layer a retention layercoupled to said reflecting layer, said retention layer having a glasstransition temperature to permit said protrusion to deform saidretention layer thereto; and (e) cooling said retention layer below saidglass transition temperature while so deformed to fix said protrusion insaid expansion layer.
 2. A method in accordance with claim 1 in whichsaid retention layer is optically coupled to expansion layer and isbetween said expansion layer and said reflective layer.
 3. A method inaccordance with claim 1 in which said retention layer is partiallyabsorptive of light of said preselected wavelength incident thereupon.4. A method in accordance with claim 1 in which step (e) is achieved inpart by diffusing energy from said retention layer into a heatabsorptive layer which dissipates heat at a rate faster than saidexpansion layer.
 5. A method is accordance with claim 1 in which saidexpansion layer absorbs at least about 40% of light at said preselectedwavelength incident thereupon.
 6. A method in accordance with claim 1 inwhich said expansion layer absorbs less than about 90% of light at saidpreselected wavelength incident thereupon.
 7. A method in accordancewith claim 1 in which said expansion layer absorbs from about 50% toabout 85% of light at said preselected wavelength incident thereupon. 8.A method in accordance with claim 1 in which said preselected wavelengthis defined as a first preselected wavelength and said retention layerabsorbs light at a second preselected wavelength to which said expansionlayer is substantially transparent.
 9. A method for recording data on anoptical data storage medium mounted on a rigid substrate, comprising:(a)passing a beam of light at a first preselected wavelength through saidsubstrate; (b) passing light emerging from said substrate through anexpansion layer optically coupled thereto which absorbs from about 50%to about 85% of light of said first preselected wavelength incidentthereupon and is substantially transparent to light at a secondpreselected wavelength and which expands with increasing temperature;(c) passing light transmitted through said expansion layer through aretention layer coupled thereto which absorbs light at said secondpreselected wavelength incident thereupon, said retention layer having aglass transition temperature substantially above ambient temperature;(d) reflecting light transmitted through said retention layer off of adeformable reflecting layer and back through said retention andexpansion layers, to expand said expansion layer with heat generated bylight absorbed thereby combined with light absorbed in step (b) andthereby form a protrusion on the surface of said expansion layer facingaway from said substrate and to raise the temperature of said retentionlayer above said glass transition temperature by conduction from saidexpansion layer and direct heating to permit said protrusion to deformsaid retention layer to conform thereto; (e) cooling said retentionlayer below said glass transition temperature while so deformed byabsorbing heat therefrom into a heat absorptive layer which dissipatesheat at a rate faster than said expansion layer, to fix said protrusionin said expansion layer.